1. Field of the Invention
The present invention concerns a process for fabricating hollow structures by depositing a thermoplastic material in powdered form on an exterior surface of a mold form and an apparatus for carrying out this process.
2. Description of the Prior Art
Hollow structures such as noncorroding fluid container liners have previously been formed from a variety of thermoplastic materials, such as polyethylene, polypropylene, nylon, Celcon, and Hytrel. One method of fabrication, called the "roto-casting" process, involves placing a predetermined quantity of thermoplastic material in powdered form inside a split-mold, inserting the mold and powdered thermoplastic material combination into an oven and rotating the combination about one or more axes at a temperature approximately the same as the melting temperature of the powdered thermoplastic material. The powdered thermoplastic material splashes freely within the mold and adhers to the inner surface of the mold as the surface temperature approaches the melting point of the thermoplastic material. The mold is removed from the oven after the thermoplastic material has been deposited over the entire inner surface of the mold and the resulting structure is removed from the interior of the mold when cooled.
This process, however, suffers from a number of disadvantages. Chief among these is an inability to control the wall thickness of the resulting thermoplastic material structure. Pinholes often occur in portions of the resulting structure due to poor distribution of the thermoplastic material within the mold during the heated rotation step. Alternatively, excessively thick and heavy structures result from attempting to achieve a minimum wall thickness over the entire surface of the structure. In addition, the interior surface of the resulting structure commonly has a rough or uneven surface. This rough surface can cause problems with fluid purging procedures when the resulting thermoplastic structure is used as a fluid container. It is also commonly difficult to adhesively bond most of the thermoplastic materials best suited for use as fluid container liners due to their noncorrosive characteristics. This bonding problem can be further exacerbated by an often necessary practice of coating the inside of a roto-cast mold with a release agent before inserting the powdered thermoplastic material. Because of this bonding problem, difficulties are frequently encountered in fabricating a fluid container having an exterior shell bonded to an inner liner formed from thermoplastic materials by the roto-casting process.
Attempts have been made to avoid the problems encountered in practicing the roto-casting process. One process, described in U.S. Pat. No. 3,009,209, involves spraying a liquid reactive mixture capable of forming a solid polyurethane plastic onto a rotating mandrel form. While permitting a more careful monitoring of wall thickness, this process is limited to liquid reactive materials which can be spray deposited through an aerosol nozzle onto a mandrel form while still in a liquid state. Similarly, a process described in U.S. Pat. No. 3,379,591 involves forming tubular articles by spray depositing resin-glass compositions and incorporating cut lengths of filament roving onto a rotating mandrel form. This process is also limited to those resin-glass compositions which can be deposited from an aerosol nozzle while still in a liquid state.
Thus, there exists a need for a process for forming hollow structures from a powdered thermoplastic material permitting careful monitoring of the wall thickness during fabrication. There further exists a need for a process for forming a hollow structure from powdered thermoplastic plastic materials to which external container shells and the like can be more easily bonded and which have a relatively smooth inner wall. In addition, there exists a need for an apparatus by which this process can be practiced.